CODEWITHSUNDEEP
pythonpython-3.xassignment-operator
planetp
planetp

Reputation: 16065

Understanding Python multiple assignment

Consider this assignment statement example:

>>> x, y = x[y] = {}, None
>>> x
{None: ({...}, None)}
>>> y
>>>

What is the value assigned to x and how does this assignment work?

Upvotes: 1

Views: 62

Answers (1)

chepner
chepner

Reputation: 530960

The statement assigns the value on the far right to each target to its left, starting at the left. Thus, it's equivalent to

t = {}, None
x, y = t
x[y] = t

So, t starts out as a tuple consisting of an empty dict and the value None.

Next, we unpack t and assign each part to x and y: x is bound to the empty dict, and y is bound to None.

Finally, we can assign the tuple to x[y] as well, since we just defined x and y. The key None is added to the dict referenced by x, and its value is the original tuple. Thus, we've made x[y] refer to x itself: a cycle!

Python can detect this cycle, so it shows the dict as {...}, rather than trying to expand it infinitely to {None: ({None: ({None: ....

Upvotes: 2

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